The present invention relates to the field of magneto-optic recording and playback. More particularly, it relates to improvements in apparatus for playing back digital information which has been previously recorded in a magneto-optic (M-O) recording element.
The method of optically reading magnetically recorded digital information by use of the magneto-optic Kerr effect is well known. Such method basically involves the steps of scan-irradiating the recording element with a continuous-wave beam of plane-polarized radiation (e.g. emanating from a c-w laser), and detecting small clockwise or counterclockwise rotations, typically on the order of 2 degrees or less, in the plane of polarization of the reflected beam. The direction of such rotation is determined by the orientation (either up or down) of the irradiated, vertically-oriented magnetic domains representing the recorded information.
A conventional M-O recording element comprises a relatively thick (e.g. 1.2 millimeters) transparent substrate which supports a relatively thin (e.g. 100 nanometers) layer of magneto-optic recording media. The magneto-optic layer may comprise any one of a variety of compounds which exhibits a relatively strong Kerr effect; presently preferred materials include thin films of a transition metal/rare earth alloy. During read-out, the magneto-optic media is irradiated through its transparent substrate. Typically, the substrate is in the form of a disk which is adapted to be rotated about its central axis.
In reading out information recorded along nearly-concentric data tracks on a M-O disk, an optical playback head is moved radially across the disk. The playback head is usually "self-contained" in that it includes all of the components, both optical and electronic, required to produce an electrical signal representative of the recorded information. The viability of a self-contained optical head has resulted from the development of small, light-weight, and efficient laser diodes, photodetectors, and associated electronic components. But no matter how light-weight and small these components, their mass can still impose limitations on the speed and accuracy with which the head can be moved from one position to another in order access information on different data tracks. Moreover, having the laser source aboard the optical head can lead to optical alignment, thermal, and optical noise problems.
In U.S. Pat. No. 4,626,679 issued on Dec. 2, 1986 in the name of Kuwayama et al, there is disclosed a variety of optical head configurations in which a laser source and a photodetector package are optically coupled to a movably mounted lens by one or more flexible optical fibers. This "split head" arrangement allows the more bulky and heavier head components to remain stationary relative to the disk while only the objective lens used to focus the readout beam on the disk is moved relative to the disk surface. In FIG. 10 of this patent, there is disclosed a magneto-optic read-out apparatus in which read-out light emitted from a stationary laser is optically coupled to a movably mounted beam splitter and objective lens by an optical fiber. The beam splitter and objective lens move radially with respect to the disk to focus read-out light on any one of the concentric data tracks on the disk. Light reflected from the disk returns through the objective lens, strikes the beam splitter and then is redirected in a direction 90 degrees with respect to the incident light. A second optical fiber is positioned to receive the reflected light and to transmit such light toward a polarizing beam splitter which serves to separate the beam into its two polarization modes (TE and TM). A pair of photodetectors are positioned to detect the intensity of the TE and TM modes, and the respective outputs of the two photodetectors are subtracted to derive a signal representative of the Kerr rotation angle produced by the recorded information.
While the magnetic-optic playback apparatus disclosed in the above-mentioned patent affords those advantages inherent in the removal of the laser and photodetecting elements from the movable portion of the optical head, it is disadvantageous in certain respects. For example, by requiring two optical fibers, one for transmitting laser energy to the disk and another for transmitting reflected energy from the disk to the photodetectors, it presents optical alignment problems in that both fibers require precise optical alignment with collimation and condensing lenses. Moreover, the need for two fibers necessitates the use of a relatively bulky beam-splitter cube which must be mounted on the movable portion of the head to separate the incoming and reflected beams.